1. Field of the Invention
The invention relates to a fuel cell system.
2. Description of the Related Art
Conventionally, the fuel cell is formed with a plurality of unit cells that is connected in series. Each unit cell generates electric power from a fuel gas containing hydrogen that is supplied to the anode, and an oxidant gas containing oxygen that is supplied to the cathode. The amount of supply of a reactant gas (i.e., the fuel gas or the oxidant gas) needed for each unit cell depends on the required generated current (i.e., load current). Therefore, in a fuel cell system, the amount of supply of the reactant gas is controlled in accordance with the load current so as not to become insufficient.
However, for some causes, a reactant gas in one or more unit cells may sometimes become lack of the amount of supply with respect to the needed amount. Examples of such cases include the case where a gas channel in a unit cell is occluded by the water produced due to the electric power generation, or the case where water is frozen in a gas channel and therefore occludes the gas channel. If the reactant gas becomes lack of the amount of supply with respect to the needed amount, the required current cannot be caused to flow solely by the power generation reactions of the reactant gases (i.e., normal power generation reactions). However, since the unit cells are connected in series, even the unit cell being lack of the supply of the reactant gas is required to cause the flow of the same amount of electric current as the other normal unit cells. Therefore, when the current is forced to flow in the unit cell lacking in the supply of the reactant gas, the following abnormal chemical reactions may occur.
A unit cell lacking in the supply of the fuel gas needs to extract electrons from the anode despite the absence of hydrogen. In consequence, there occur an oxidation reaction of water (2H2O→O2+4H++4e−), an oxidation reaction of carbon (C+2H2O→CO2+4H++4e−) and an elution reaction of Pt (Pt→Pt2++2e−) as well as an oxidation reaction of an electrolyte component, etc. On the other hand, a unit cell lacking in the supply of the oxidant gas needs to receive, at the cathode, electrons despite the absence of oxygen. In consequence, there occurs a phenomenon in which protons move from the anode side to the cathode side through an electrolyte membrane, and recombine with electrons. That is, a so-called “hydrogen pump phenomenon” occurs.
In particular, the abnormal chemical reactions due to the supply shortage of the fuel gas cause damage and degradation of the MEA (Membrane Electrode Assembly). Therefore, in the fuel cell system, it is important to detect the supply shortage of the fuel gas during an early period. Such detection may be achieved by measuring the voltage of each unit cell. The anode potential of the unit cell lacking in the supply of the fuel gas increases in response to the abnormal chemical reactions, and becomes higher than the cathode potential. That is, a so-called “reverse potential phenomenon” occurs. Therefore, via the voltage (i.e., the reverse potential) monitoring of each unit cell, it may be determined whether the supply the fuel gas is lacking.
According to fuel cell systems of related arts, if a cell voltage of any unit cell falls below a pre-set threshold voltage, a predetermined voltage recovery process is performed; for example, the load current is set to a lower value, or the power generation is temporarily stopped. Technologies related to the reverse potential of a fuel cell are described in, for example, Japanese Patent Application Publication No. 2006-147178 (JP-A-2006-147178), Japanese Patent Application Publication No. 11-67254 (JP-A-11-67254), Japanese Patent Application Publication No. 2004-30979 (JP-A-2004-30979), Japanese Patent Application Publication No. 2006-73501 (JP-A-2006-73501), Japanese Patent Application Publication No. 2006-49259 (JP-A-2006-49259), and Japanese Patent Application Publication No. 2004-241236 (JP-A-2004-241236).
When the fuel cell starts up at low temperature, in particular, starts up at a temperature below the freezing point, gas channels may be occluded by ice or water, and thereby the fuel gas may be insufficiently supplied to the fuel cell. To remove the ice or water occluding a gas channel, the self-heating of the fuel cell may be utilized. To increase the self-heating of the fuel cell, it is effective to cause the amount of supply of the oxidant gas to be in a shortage state. Therefore, by supplying the oxidant gas less than the required amount of the oxidant gas from the load current, the over-voltage of the cathode may be increased and therefore the self-heating of the fuel cell may be accelerated.
When the oxidant gas is supplied insufficiently, the cathode potential greatly falls from a steady operation potential. Furthermore, the resistance of the MEA becomes greater at low temperature, so that the voltage loss due to the resistance of the MEA also becomes conspicuous. As a result, the cell voltage may sometimes become negative, as in the case of the supply shortage of the fuel gas. However, unlike the increase in the anode potential due to the supply shortage of the fuel gas, the decrease in the cathode potential due to the supply shortage of the oxidant gas is allowable. If this is not allowed, the warm-up of the fuel cell cannot be accelerated, so that the startability of the fuel cell at low temperature will decline.
However, only the cell voltage may be measured practically, and thereby each of the anode potential and the cathode potential cannot be practically measured unless a reference electrode is provided. Therefore, according to the fuel cell system of the related arts, when it is forced to insufficiently supply the oxidant gas for the purpose of warming up the fuel cell system at low-temperature startup, it cannot be determined whether the reverse potential of a unit cell resulted from the increase in the anode potential. Hence, the threshold voltage used for the determination as to whether to start the aforementioned voltage recovery process cannot but be set to a higher value for the preferable protection of the membrane electrode assemblies.